The invention relates to integrators for use in lithographic projection apparatus comprising:
an illumination system for supplying a projection beam of electromagnetic radiation having a wavelength of less than about 50 nm;
a first object table provided with a first object holder for holding a mask;
a second object table provided with a second object holder for holding a substrate; and
a projection system for imaging an irradiated portion of said mask onto a target portion of said substrate.
More particularly, the invention relates to such a device in which the electromagnetic radiation is extreme ultra-violet light (EUV), typically with a wavelength below about 15 nm. An example of a wavelength in the EUV region which is gaining considerable interest in the lithography industry is 13.4 nm, though there are also other promising wavelengths in this region, such as 11 nm, for example.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam, and such elements may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. In addition, the first and second object tables may be referred to as the xe2x80x9cmask tablexe2x80x9d and the xe2x80x9csubstrate tablexe2x80x9d, respectively. Further, the lithographic apparatus may be of a type having two or more mask tables and/or two or more substrate tables. In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto an exposure area (die) on a substrate (silicon wafer) which has been coated with a layer of energy-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies which are successively irradiated via the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each die is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally less than 1), the speed V at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO97/33205, for example.
Until very recently, lithographic apparatus contained a single mask table and a single substrate table. However, machines are now becoming available in which there are at least two independently moveable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO98/28665 and WO98/40791. The basic operating principle behind such multi-stage apparatus is that, while a first substrate table is at the exposure position underneath the projection system for exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge a previously exposed substrate, pick up a new substrate, perform some initial measurements on the new substrate and then stand ready to transfer the new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed; the cycle then repeats. In this manner it is possible to increase substantially the machine throughput, which in turn improves the cost of ownership of the machine. It should be understood that the same principle could be used with just one substrate table which is moved between exposure and measurement positions.
In the case of the current invention, which encompasses electromagnetic radiation in the EUV range, the projection system will generally consist of an array of mirrors, and the mask will be reflective; see, for example, the apparatus discussed in WO 99/57596 (P-0111). The radiation in this case can be produced by various known means, such as:
by suitable laser-irradiation of a gas, liquid or solid;
on the basis of a plasma source;
with the aid of an undulator/wiggler placed around the path of an electron beam in a synchrotron or storage ring.
An example of an illuminator comprised in the illumination system and suitable for use with such radiation is described in European Patent Application no 98204237.6 (P-0122), whereas a suitable condenser for use with EUV is described in European Patent Application no 00300784.6 (P-0129).
In general, it will be desirable to incorporate an integrating element in the illumination system, which element serves to improve the intensity uniformity throughout the cross-section of the projection beam prior to the mask. In the case of UV lithography, such an integrating element (xe2x80x9cintegratorxe2x80x9d) may comprise a so-called fly-eye lens, or a refractive bar (such as a quartz rod). However, such integrators are not suitable for use with radiation in the EUV range, and alternatives have therefore been sought. To date, the use of alternatives such as ripple plates, multilayer mirrors and wigglers has been proposed.
It is an object of the invention to provide an alternative integrator for use with electromagnetic radiation having a wavelength of 50 nm or less, and particularly for use with EUV.
These and other objects are achieved in a lithographic projection apparatus comprising:
an illumination system for supplying a projection beam of electromagnetic radiation having a wavelength of less than about 50 nm;
a first object table provided with a first object holder for holding a mask;
a second object table provided with a second object holder for holding a substrate;
a projection system for imaging an irradiated portion of said mask onto a target portion of said substrate; and
an integrating element disposed in the path of the radiation in the illumination system, the integrating element comprising a hollow waveguide.
The inventors have demonstrated that such an integrator can have a surprisingly high efficiency and transmission, and lends itself to many varied applications. Preferential embodiments of the invention are specified further in the claims, and a number of these embodiments are elucidated below in the embodiments and figures.
The invention also relates to a method of manufacturing a device using a lithographic projection apparatus comprising:
an illumination system for supplying a projection beam of electromagnetic radiation having a wavelength of less than about 50 nm;
a first object table provided with a first object holder for holding a mask;
a second object table provided with a second object holder for holding a substrate; and
a projection system for imaging an irradiated portion of said mask onto a target portion of said substrate; the method comprising the steps of:
providing a mask bearing a pattern to said first object table;
providing a substrate having a radiation-sensitive layer to said second object table;
irradiating portions of the mask with said projection beam; and
imaging irradiated portions of the mask onto target portions of the substrate;
characterized in that, prior to being directed onto the mask, the projection beam is passed through an integrating element which comprises a hollow waveguide.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices (dies) will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0- 07-067250-4.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9cexposure areaxe2x80x9d, respectively.
In the present document, the invention is described using a reference system of orthogonal X, Y and Z directions and rotation about an axis parallel to the I direction is denoted Ri. Further, unless the context otherwise requires, the term xe2x80x9cverticalxe2x80x9d (Z) used herein is intended to refer to the direction normal to the substrate or mask surface, rather than implying any particular orientation of the apparatus.